A normally-closed switch is a switch designed to permit the flow of electricity through the switch when that switch is in a "normal" or non-distorted configuration. Such switches are useful in a wide variety of safety and security applications, for example, for detecting the movement of large pieces of furniture or valuables. For example, a safe may be placed on a pressure actuated normally-closed electrical switch thereby interrupting the electrical circuit which could typically be connected to an alarm system. If the safe was removed from that location, the switch would return to its "normally" closed position, thereby completing the electrical circuit and activating the alarm. Other applications include the placement of normally-closed switches in industrial work areas where the switches are used to detect the presence of personnel in close proximity to dangerous machinery. In these applications, the switches can be utilized to disrupt the electrical circuitry which powers the dangerous equipment thereby deactivating the equipment when a person moves dangerously close to that equipment. Other applications have and will be appreciated by those skilled in the art for pressure-sensitive normally-closed electrical switches.
Included among the normally-closed electrical switches known in the art are those disclosed in U.S. Pat. No. 3,553,404 to Koenig, 3,717,735 to Koenig, and 4,296,283 to Koenig et al.
U.S. Pat. No. 3,553,404 discloses a normally-closed pressure switch having a pair of curved contact members disposed between parallel stiffener plates and separated at the edges by insulators. When a force is applied to the stiffener plates, the stiffener plates cause the curved contacts to straighten thereby breaking the electrical contact between the curved contacts.
U.S. Pat. No. 3,717,735 discloses a normally closed switch having two conductor strips with transverse curvature. This normally-closed switch is "flex sensitive" such that when the switch is flexed, the strips lose their transverse curvature and the contact between the conductor strips is broken.
U.S. Pat. No. 4,296,283 similarly discloses a normally-closed "wafer" switch having outer stiff plates, inner contact plates having transverse curvatures wherein the contact plates are aligned so that their axes of curvatures are perpendicular, and an insulating plate disposed between the curvature plates.
In addition to the normally closed electrical switches discussed above, many types of normally-open pressure sensitive switches are known in the art. Included among such normally open switches is the tape switch disclosed in U.S. Pat. No. 2,770,696 to Koenig which is adapted to be actuated by the weight of a person or vehicle passing over it.
The above-described normally closed electrical switches are, for practical purposes, limited in size and design. Unlike the novel switch described herein, the typical normally-closed switches referenced above were designed to open upon the application of force to the entire length of the strip as opposed to pressing at any point.